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Definition

noun

A metabolic pathway where CO₂ is converted to 3-phosphogylycerate, the first stable intermediate organic compound containing three carbon atoms.

Supplement

The C₃ carbon fixation pathway is in fact the initial phase of Calvin cycle. It starts with the enzyme rubisco catalyzing the carboxylation of Ribulose-1,5-bisphosphate by CO₂ producing a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, which splits instantaneously into two molecules of the more stable 3-phosphogylycerate, an organic compound containing three carbon atoms (hence the name C₃).

Plants that solely depend to C₃ pathway for carbon fixation are faced with the negative effects of photorespiration, such as the wasteful loss of CO₂. Photorespiration occurs under conditions of drought, high temperatures and low nitrogen or CO₂ concentrations. These conditions cause the stomata to close in an attempt to prevent excessive water loss. The closure of stomata increases O₂ levels, and the enzyme rubisco reacts with O₂ instead of CO₂, consequently losing CO₂ instead of fixing CO₂.

Word origin: from the intermediate compound, which contains three carbon atoms, hence the name C₃.

Compare: C4 carbon fixation pathway, crassulacean acid metabolism.
See also: C3 plant.